Histopathology (especially subtyping and grading of tumors) and immunohistochemistry (IHC) play a central role in the experimental designs of nearly every component of this Program Project. The purpose of this Core is therefore to provide large-scale histopathological and IHC evaluation of premalignant and malignant breast lesions to five Projects. Overall this effort will involve cutting, staining, and (most important) interpreting nearly 26,000 slides, and we feel that the core is necessary to ensure cost efficiency and uniformity of results for this immense workload. We have the necessary laboratory space, equipment, tumors, and especially human resources to make this centralized approach work successfully and productively -- during the previous funding period, this core component analyzed 16 separate markers and over 20,000 slides for Program Project investigations.